Separating ethanol & water

Separating ethanol from water

Separation methods

Fermenting sugars to produce ethanol yields a fermentation broth (or beer) with about 12% ethanol. This has been traditionally separated by distillation. However this is energy intensive so other methods should be looked at:

The various methods possible are:

  1. Boil off the ethanol in a series of steps, each one becoming more pure.
  2. Add a salt that dissolves in  water or ethanol producing a heavy layer.
  3. Use a solvent which dissolves ethanol but not water:
  4. Use a membrane for reverse osmosis.
  5. Blow insoluble gas through the mixture: the ethanol will preferentially vaporise leaving ethanol lean water. Ethanol can then be condensed from the gas.
  6. Find a solid which preferentially adsorbs either ethanol or water, leaving the other behind.
 

Fermentation

In the fermentation process, yeast Saccharomyces cerevisiae converts half the sugar to CO2, and half into ethanol. For years this was the main source of CO2 for fire extinguishers, and sparkling drinks.

When ethanol in the fermentation broth reaches 12-14%, it kills the yeast and stops the process.

Yeast does not ferment 5 carbon sugars.

 

Distillation of ethanol

Ethanol cannot be simply boiled out of water because some water is always included in the vapour. Each time a mix of ethanol and water is boiled, the vapour is richer in ethanol that in the liquid. This vapour must be condensed and then boiled again to produce richer vapour again.

This diagram shows how it works. (It is in mole % because it works better that way). A liquid of about 17 mole% ethanol will produce a vapour of 50 mole% ethanol Then this when condensed and boiled again, it will produce a vapour of 63%, this in turn will produce a vapour of 68%, and so on, approaching but never quite reaching 95.6% ethanol by volume..

At 95.6% ethanol by volume, an azeotrope is formed. This is a mixture that boils at 78.1oC, a temperature lower than either the ethanol 78.5 atoC or the water at 100oC, so more boiling will get nowhere.

Because it takes so much energy to approach 95%, it is usual to stop at 92% ethanol. To reach the azeotrope would take an infinite number of stages or plates, and an infinite amount of energy.

The water removal from the azeotrope is carried out by distillation with pentane, benzene, diethyl ether, ethylene glycol, gasoline, or absorbed by zeolite, corn meal, CaCl2, BaO, Na, or Silica Gel.

Cornmeal absorber for drying ethanol

McCabe–Thiele diagram for ethanol and wa

How a distillation column works

The "beer" or "wine", (ethanol water mix from fermentation), is fed to the lower section of the column. Here hot vapour from below bubbles up through the liquid, stripping out most of the ethanol leaving water in exchange. This repeats at every plate. The plates can be replaced with packing of various types.

 At the top, the vapour is taken off and condensed, some is removed as product, the rest is sent back down the column as reflux to strip water out of the vapour. The reflux ratio determines how pure the product is, and how much energy is used.

Ethanol distillation - the fundamentals

Distillation principles

Solar still 1991

Building a still

Energy in distillation

The problem is that the energy required to distil out the ethanol from water is about half of the energy contained in the ethanol. Coupled to the other energy required to grow and process the crop, the process may not be worth pursuing.

 

Salts

Adding salts such as KF, Na2SO4, Na2CO3, or similar, to the beer causes the ethanol to separate out as another layer.

It cost about the same as distillation.

Oak Ridge

Sulfur

Sulfur does not dissolve in water but it dissolves slightly in ethanol. The sulfur/ethanol solution sinks to the bottom as a separate layer. The ethanol is distilled off from this mixture but it is more economic as it does not require reflux.  Source

 

Liquid-liquid extraction

Ca​stor oil

Castor oil (or similar) can be added to the mix. Ethanol  dissolves in it but water does not. The oil/ethanol layer floats to form a top layer. The oil can be heated to drive the ethanol vapour off.  Source

Ionic liquids

Phosphonium-based ionic liquids (ILs) can dissolve ethanol from fermentation broth then removed from the liquid by pervaporation.

Pervaporation is a two step process: (1) permeation through the membrane (2) evaporation into the vapor phase,

Paraffin oil

Light paraffin oil is miscible with ethanol at 115oC but not at 30oC. So if the oil is mixed with the beer (fermentation broth) and heated, then the oil layer taken off and cooled, the ethanol has only 1% water.  Other oils that will do the job are hexadecane and cottonseed oil.   Source

Reverse osmosis

As water and ethanol molecules are of different sizes and shapes it is possible to force then through a very fine filter called a membrane.

Membrane solvent extraction (MSE)

This patented low-energy separation process uses a porous membrane to separate ethanol from the fermentation broth using an extraction solvent.

Source

Research - Graphene membrane

Graphene oxide membranes allow water vapor to pass through, but are impermeable to other liquids and gases. This phenomenon has been used for further distilling of vodka to higher alcohol concentrations, in a room-temperature laboratory, without the application of heat or vacuum as used in traditional distillation methods.[203] Further development and commercialization of such membranes could revolutionize the economics of biofuel production and the alcoholic beverage industry. 

Wikipedia

Schematic of separation of fermentation broth and a high-boiling extraction solvent using MSE. Right − Close up of microporous-membrane. By stacking layers of the membrane and incorporating integral channels, a high surface area for separation can be obtained.
Image courtesy of 3M

Short Path Condensation Recovery

R3Fusion’s SPaCeR™ Technology uses a focused heating process combined with a thin film process coupled with heat and mass transfer efficiencies. This innovative process has been perfected through more than twenty years of process intensification research done by R3 Fusion founder Dr. Roshan Jachuck.

Very vague on details this process claims to be the answer to everything.  Website

The process creates a thin-film of fluid running along side the inside walls of a heated cylinder. The water in the thin film is rapidly evaporated to form water vapour through a combination of heat and low pressure. Water vapours move off the heated surface into a gap between the heated cylinder and a condenser where the vapour condenses as pure water and flows down the condenser wall into a collector. There is a stream of distilled water and a stream of concentrated water leaving the system. Source

(I must be missing the point, I cannot see how this process will remove ethanol from water in one stage only. The person writing this website is either trying to conceal the process, or doesn't understand it. John Davis - Editor)